Since it was first identified in 2007, deadly equine multinodular pulmonary fibrosis (EMPF) has been reported in numerous horses across North America and Europe. While still considered a rare disease, EMPF appears to be related to a very common one—equine herpesvirus (EHV)—and early treatment appears to be the main hope for survival.

Specifically, it’s the “gamma” herpesviruses that seem to be involved in EMPF development, explained Bianca Schwarz, PhD, DVM, Dipl. ECEIM, head of the Internal Medicine Service in the Equine Clinic of Altforweiler, Germany, and a former researcher at the Equine Clinic of the University of Vienna in Austria. There are two known gamma herpesviruses in horses: EHV-2 and EHV-5. These EHV forms often go unnoticed in horses, as the infection can be subclinical (i.e., it doesn't cause outward clinical signs). But if signs do occur, they are usually related to the respiratory tract, causing nasal discharge or breathing difficulties, and in more severe cases fever and lethargy. There is no vaccine for EHV-2 or -5.

Here's how it all ties together: In all documented EMPF cases, the affected horses have also tested positive for EHV-5, Schwarz said. And about one-third of those horses tested positive for EHV-2, as well. Schwarz says that, even so, EMPF itself cannot be considered a “viral” disease: “At the moment we think that it is a wrong immunoreaction of certain horses to a viral pathogen,” she said.

Looking at five of the seven EPMF cases seen in two European referral clinics between late 2008 and mid-2011, Schwarz and colleagues noted two distinct forms of EMPF: “discrete” and “diffuse.” In EMPF, the lungs develop fibrosis nodules—a kind of bumpy scarring—in the alveoli. The alveoli are tiny pockets in the lungs that receive the incoming air during breathing and process its gases for the body to use. In the discrete form of EMPF, nodules can range from small to very large (up to 6.5 cm in Schwarz’s case studies) and are sharply defined in the lung tissue. By contrast, in the diffuse form the nodules are mostly small and widespread. In the current study three of the horses had the discrete form, whereas the other two had the diffuse form.

In two additional case studies, also led by Schwarz, two horses were diagnosed with the diffuse form of the disease.

Consistent with previously findings, all seven of these horses were found to be positive for EHV-5, Schwarz said. One horse was also diagnosed with concurrent T cell leukemia, which also appears to be related to EHV-5, she added. Leukemia is primarily a disease of the bone marrow, and EHV-5 appears to cause pathological changes in the bone marrow in some horses.

Only one of the seven study horses survived the disease, Schwarz said, and at 22 years old he was also one of the oldest affected. In a pioneering effort, researchers at the University of Veterinary Medicine in Vienna treated the Trakehner stallion with the antiviral medication valacyclovir, without corticosteroids. After one week of treatment, the stallion made a full recovery and was still healthy two years later. However, this successful outcome should not be attributed to the medication alone, she cautioned. The fact that the disease was caught early in its progression probably played an important role in the horse’s survival.

Clinical signs of EMPF usually include weight loss and progressive respiratory disease, especially difficulty breathing, said Schwarz. Because EMPF is so rare, veterinarians usually do not suspect it initially and often treat the horse for a bacterial disease instead. By the time EMPF is discovered, it’s typically too late to institute effective treatment.

“This disease is a terrible disease,” Schwarz said. “I don't think that these horses are in great pain, but they definitely are in great distress. Depending on the severity of lung lesions, they have less oxygen in their blood, which makes them feel like they’re suffocating.”

About the Author

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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